Starting control for electric discharge device



Aug. 19, 1947. M.'A. TowNs'END 2,425,902

n STARTING CONTROL FOR ELECTRIC DISCHARGE DEVICESl Filed .June 8. 194547 @ab (amargo 4gb 59h nventor:

}l/55b (//vsumwa/y) Mark A. Townsend,

Patented Aug. 19, v1.947

STARTING CONTROL FOR ELECTRIC DISCHARGE DEVICES Mark A. Townsend,Lubbock, Tex., ass'ignorA to General Electric Company, a corporation ofNew York Application June 8, 1943, Serial No. 490,048

This invention relates to the control of electric discharge devices, andis especially concerned with the starting of such devices. The inventionis very useful in connection with fluorescent tubes or lamps of thepositive column discharge type, and is hereinafter explained withparticular reference to lamps of this character and their usual startingarrangements, as exemplified in U. S. Patent 1,951,112, March 13, 1934,to Wels, or Patent 2,212,427, August 20, 1940, to Peters, for instance.

Ordinary tubular fluorescent lamps have electrical starting circuitswhich are energized and broken to start the discharge. Sometimes thishas to be done several times before the discharge actually starts. Forthis purpose, automatic means are commonly provided which will make andbreak the starting circuit indefinitely until starting is accomplished.

, Through deterioration in service or as the result of accident,fluorescent lamps and other discharge devices often or eventually becomeincapable of starting and normal running, so that the only effect of theautomatic efforts to start such a lamp is a continual flashing in itthat is very annoying to those in the area that receives the light ofthe flashing lamp, besides uselessly Wearing out the starter device.Usually such incapacity for starting is due to loss of electron emissionfrom one or both electrodes of the lamp, which may result fromdissipation or other loss of activating material on which theemission-depends. lamp goes on indefinitely until the power supply isshut off, or the lamp is removed from the circuit.

'I'he purpose of this invention is to obviate the undesirable flashingof defective lamps without4 ciation with several types of automaticstarter;

but the invention is also applicable in connection with other types ofstarter. In suitable forms of embodiment such as hereinafter described,the invention affords advantages of simple and rugged construction,durability and reliable performance in service, capacity for flexibleand accurate regulation of lock-out time 7 Claims. (Cl. 20D-122) i 2 toallow proper opportunity for starting the lamp, as desired, and ease andlow cost of manufacture. Still other features and advantages of theinvention will become apparent from the description of species and formsof embodiment, and from the drawings. This applicationis acontinuation-in-part of applicationv Serial No. 438,208 filed April 8,1942, by Jack R. Whiteside and Mark A. Townsend, and assigned to theassignee of this application. The present application is made to protectthe `further invention which I believe to be present in the subjectmatter set forth in the claims hereof over and beyond the claims setforth in the above mentioned joint application.

In thedrawings, Fig. 1 is a diagrammatic view of a fluorescent tube orlamp with circuit connections including the present invention, amidportion of the lamp envelope being broken out and omitted; Fig. 2 isa tilted or perspective view of one form of starting switch and lock-outdevice; Fig. 3 is a tilted orA perspective similar to The objectionableflashing of such a Fig'. 2 illustrating the lock-out in connection witha glow-switch, and slightly modified; and Fig. 4 is a fragmentary viewsimilar to Fig. 3 but illustrating a vstructural modification.

Fig. l shows an ordinary fluorescent lamp L of the positiveI columnelectric discharge type with a tubular envelope I0 having spaced apartactivated thermionic cathodes Il, Il in its ends, which may be speciallyheated cathodes of usual coiled filament type activated with coatings ofrefractory oxides, such as a mixture of alkaline earth oxides includingbarium and strontium oxides, and are shown connected across apowersupply circuit P including the usual ballast I4, which also servesas a starting inductance, and the manual make-and-break switchv l5. Theenvelope I 0 may contain a low-pressure atmosphere of starting gas, suchas argon at a pressure of 2 to 5 mm. of mercury, and also a vaporizableand ionizable working substance such as mercury. A supply of mercury,which may exceed the amount that will vaporize during operation of thelamp L, is indicated by a drop I1 inside the envelope I0, and aninternal coating of fluorescent material or phosphor i8 on theenvelopeWalls is also indicated. A starting and electrodeheating circuit H isshown connected across the circuit P through the illamentary cathodes ll, l I, with a starting switch S included therein, which may be ofthermal'type. A capacitor or condenser |9 of suitable capacity (e. g.,0.007 microfarad) may be connected across the switch, to minimize radiointerference as well as arcing being both turned away from the plate 2|.

when the switch opens and breaks the circuit H.

The general mode of operation in starting the lamp L with this circuitarrangement is that when the switch I is closed to turn on the lamp L,the switch device S permits flow of current through the circuit H andthe cathode filaments Ii, Il in series therein for a sulcient length oftime to preheat the cathodes to an adequate emissive temperature, andthen suddenly opens the circuit H: and the resulting voltage kickbetween the cathodes Il, -Il suillces to initiate discharge betweenthem, or, in other words, starts the lamp.

As shown in Fig. 2, the switch device S has as its base an insulativedisc 20, and also includes ansupright insulative frame plate 2l.Underneath the base disc 20 are shown binding post connectors 22, 22 towhich the wires of the circuit H are connected, and their reduced Shanksextend up through the disc and are headed over to secure them. Theswitch device S is shown in Figs. 1 and 2 Aas a thermal switchcomprising switch members 24, 25 that are connected to opposite sides ofthe starting circuit H through the connectors 22, 22 and coact in makingand breaking this circuit. The switch members 24, 25 may consist ofslender, approximately upright thermostatic bimetal strips coacting asboth temperature-responsive elements and current-carrying contactmembers, and are shown in Fig. 2 as mounted on one side of the plate 2|,at opposite ends thereof, to extend toward one another. Contact pieces26, 26 (as of coin silver Wire) are shown welded to the coacting freeinner ends of the members 24, 25, and may extend at right angles to oneanother. The members 24, 25 are shown as bent transversely, channelfashion, near their anchored outer ends, these bends 2l, 21

Adjacent the thermostat members 24, 25, about concentric with theirbends 21, 21, there are resistance heaters 28, 29 for heating thesemembers. The circuit connections of these heaters will be more fullydescribed hereinafter;` for the present it suiilces to say that theheater 28 for the member 24 is connected across the circuits H and P inparallel with the members 24, 25, so as to respond essentially to theenergization and voltage of the main discharge circuit P duringstarting, while the heater 29 for the member'25 is connected in thecircuit H in series with the members 24, 25, so as to respond to theenergizetion of this starting circuit H and to the current flow therein.'I'he thermostatic switch members 24, 25 may be arranged to flex in thesame direction (upward in Fig. 1, and toward the right in Fig. 2) whenheated, and vice versa. A sheet metal cover, casing, or can 30 having aninsulative lining for its cylindrical wall may enclose the whole devicefrom the base 20 up, and is shown with its end wall apertured at 3i. Theheater 2B, which is preferably of high resistance, has one end connectedby a lead 32 and a lead 33 from member 24 to one of the connectors 22,while its other end is normally connected by a lead 34 and other meanshereinafter described to the other connector 22. 'One end of the heater29 is connected'by a lead 35 to the lead 34, while its other end lsconnected by a lead 36 to the member 25, and is thus also connectedthrough the switch S (when closed) to the connector 22 to which heater28 is more directly connected. The heater 28, which is subject andresponsive to the high ballast output voltage when the switch opens, ispreferably of very high resistance, such as 80,000

to 100,000 ohms, for example, in order to prevent it from seriouslylowering the voltage availablel enough to close under the normaloperating voltage of the lamp. The heater 28, through which flows thecathode preheating current in the starting circuit H when the switch Sisclosed, is preferably of low resistance, such as 3 ohms, in order tominimize its energy consumption. Any suitable provision may be made tocause theheater 29 to heat the thermostat 25 and to cool off ratherslowly, such as enclosing this heater 29 in a n0- nex glass sleeve 38,so as to give it a high heat capacity. On the other hand, the heatingand cooling time of the heater 28 may be minimized, as by blackening itso as to lower its heat capacity.

When the switch I5 is closed for a cold start, line voltage is appliedto energize the circuits P, H and the heater 28, and the thermostatmember 24 is heated and ilexes (upward in Fig. 1, and to the right inFig. 2) into contact at 26 with the member 25, closing the switch S andfully energizing the circuit H and the heater 29. Closing of the switchS short-circuits heater 28, which is thus de-energized and starts tocool oi, as well as thermostat 24. The current flow throughl the cathodecoils l I, Il and the heater 29 preheats the cathodes and also heats upthe thermostat member 25, which fiexes (upward in Fig. 1, and to theright in Fig. 2) out of contact with the member 24, breaking the circuitH. This opening of the starting circuit H is a quick, sharp action,giving a maximum voltage kick which is generally effective to start thedischarge in the lamp L, if it is a good lamp. When the switch S opensin this manner, the heater 29 and the thermostat 25 are de-energized,and the de-energized thermostat 25 cools and uniexes. The discharge inthe lamp L shunts or short-circuits and de-energizes the heater 28 (inview of its high resistance) sulciently to prevent it from reclosing theswitch S. Ii this cycle of operation of the switch S fails to start thelamp L, it may be repeated one or more times.

In accordance with the present invention, means of control are providedfor determining the condition of the circuit H as to continuity in sucha way that after reasonable efforts of the starter S to start the maindischarge, an opencircuit condition of circuit H that prevents ashing ofthe lamp L is established and maintained, independent and regardless ofany further action or operation of starter S, indeed, the ordinaryoperation of starter S- may be suspended. For this purpose, automaticcontrol means may be employed whose period of operation (as ilxed bydesign and adjustment) is long enough l to include whatever number ofattempts by starter S may be considered reasonably worth-while, e. g., aperiod of about a minute under favorable voltage conditions, embracingsome ve to ten attempts, say. 'I'he action of this automatic controlneed not be an arbitrary matter of xed adjustment, however, but can bemade responsive vto the actual attempts of starter S to stant the maincircuit P, I may employ an independent mechanical motor Yor source ofenergy, such as a spring brought into action in response to theenergization and voltage of this circuit during starting.

For this purpose, there is shown in Figs l and 2 a self-opening switch Xconnected in series with the automatic starter S. This switch Xcomprises as one switch member a straight, ap-

proximately vupright thermostatic bimetal strip riveted to the base 29at 42. For heating the thermostatic member 40, there is a special heater43 consisting of fine resistance wire coiled around the member 40 nearits foot 4|, over a thermoinsulative wrapping of sheet asbestos 44 whichslows the transfer of heat to the/member 40 and gives it somewhatgreater heatcapacity, thus retarding the action of the switch X ascompared with that of the switch S. The upper free end of the thermostat40 is bent laterally at 45 to coact as a latch and contact with acontact and latching head or plunger 46 that is mounted to slide alongfixed longitudinal support and guide means in the form of a rod 41upstanding from one of the connectors 22 alongside the starter S. Theheadl46 has aconical downward bevel surmounted by an annularlatch-engaging contact shoulder 48. Around the upright `guide rod 41there is a switch-opening helical compression spring 49 that actsagainst the head 46 to yieldingly urge it upward toward the upperapertured casing end. As an operator for the switch X, the head 46 hasan insulative shouldered handle 50 that works or moves inward andoutward through the hole 3| in the top or end wall of the casing 36.

When the switch X is set in closed condition by engagement of latch 45with head 46 as shown in Fig. 2, it completes the starting circuit Hfrom one connector 22 through heater 43 and automatic starter S to theother connector 22. Normally, wherrthe lamp L controlled by the starterS is a good one, the contact head 46 is held down or retained as shownby the latch 45 of the thermostat 40 that extends along adjacent the rod41, and the handle 50 protrudes only slightly through the top of thecasing 30, ras shown in Fig. 2. But when the lamp L is a bad one thatcannot be started, the accumulation of heat in the thermostat 40 and itsheat-retentive lagging 44 eventually heats up this thermostat 40 untilit bends back away fromthe head 46 and releases it, whereupon the head46'and the handle 50 fiy up until stopped by engagement of the shoulderon the handle 50 with the top of the casing 30, through which the handlethen protrudes very conspicuously. This openingof the switch X opens thestarting circuit H regardless of the switch S, so that further automatice'orts to start the lamp become impossible until the switch X isreclosed by hand. The switch X can be reclosed and relatched by simplypushing down on the handle or button 50, causing the bevel ofl .-6portion below the shoulder extends through the part 55 and an insulativespacing washer 61 between the part 55 and the disc 20. Unlike the rod 41in Fig. 2, the rod 41a is not attached to one of the connectors 22, butis separately and independently riveted through the disc 20. Thethermostat foot 4|a has tangs 58, 58 that clamp around the'portionl ofthe part 55 between its outer end and an opening 59 in it near thewasher 51. The latch and contact head or plunger 46a is annularlygrooved at 48a above its conical portion to provide a shoulder forengaging the latch 45.

As here shown, the upper end wall of the casing 39a is reentrantlydished, and its operatorthe head 46 to force back the thermostat member40 and latch under its latch 45 at 48.

The device illustrated in Fig. 3 differs from that in Figs. 1 and 2inthe substitution of a glowswitch W of ordinary type for the thermalswitch S described above, as well as in certain features of the switchX. Instead of being attached directly to the base disc 20, thethermostat 40a is attached to an insulative part or block 55 that ismounted on the guide and support` rod 41a and projects laterally fromit. For this purpose, the rod 41a is flanged or shouldered at 56, andits hole 3|a. is centrally located at the bottom of the external recess.The operator 50a is shown as a separate insulativebutton loose in theopening 3|a and having an annular shoulder to engage behind the edge ofthe opening and limit protrusion of the button. Owing to the amount ofroom required in the casing 30a for the glowswitch W, the guide rod- 41ais eccentrically located therein some distance out of line with the axisof the casing and of the hole Sla and button 50a; but the head orplunger46a has a lateral portion extending under the button tov one side of therod substantially into'line with the opening 3|a, and preferablyengaging the button 50a axially of the latter and square with the buttonand casing axes, so that there is no eccentric thrust tending to cock orbind the button in its hole 3|. Instead of extending the top flange 60of the head 46a to a Width suillcient for this purpose, the head 46a maybe supplemented with a double wire loop 6 having its ends extendedlaterally across the top surface of the head 46a and bent downward. andback to hook through diametrally opposite notches 62, 62 in the flangeand under the latter. The wire of the loop 6| is made suciently stiffand springy to allow of springing the ends apart to engage them in the.notches 62, 62 in this manner, as well as to susend engages the plunger46a) may be connected to one connector 22, one end of the coiledresistance heater 43a may be connected to the thermostat foot 4 la andits other end to one glowswitch lead 63, and the other glow-switch lead64 may be connected (through an insulative sleeve or covering 65) to theother connector 22. 'I'he condenser |9 may have its leads 66, 66connected directly to the two connectors 22, 22, so that it shunts andprotects both the serially connected switches W and X, as in Figs. 1 and2.

As shown in Fig. 3, provision is made to slow the transfer of heat fromheater 43a to thermostat 40a and to give the latter greater heatcapacity, thus desirably retarding the action of switch X as comparedwith switch W while at the same time allowing heater 43al to take awattage adequate to assure operation of switch X even when the voltageat P is low owing to poor supply Voltage regulation. For this purpose,the thermostat 40a and its heater 43a are separately enclosed or evenVsurrounded (more or less completely) in thermo-insulatively separatedparallel recesses or bores in a common thermal insulator 10, which maybe made of ceramic material. 'I'his insulator 1D not only slows thetransmission of heat from heater 43a to thermostat Ilia, but alsominirnizes loss of .heat and consequent variations oi' operation due to'atmospheric temperature changes, besides preventing the heat of theglowswitch W from affecting the thermostat materially. By thusdefinitely controlling the amount of heat capacity or thermal inertiabetween the heater 43a and the thermostatic bimetal 40a, it becomespossible to have the heater 43a operate at relativelyhigh wattagewithout risk of tripping the latch 45 too soon when the supply voltageis relatively high, or of failing to trip it at all when this voltage isrelatively low. For example, a 118 volt supply may give 3.5 watts in theheater 43a when the voltage falls to 105 volts and 8.5 to 9 watts whenthe voltage rises to 130 volts, with definite assurance of adequate-opportunity for starting on high voltage and of locking out after areasonable duration of starting effort on low voltage.

For the convenience of those wishing to use the invention, particularsof a starter such as shown in Fig. 3 suitable for 30-40 watt fluorescentlamps of the usual lpositive column type are given:

The glow-switch W may be a small commercial glow-switch such as isalready in use for uorescent lamps. The thermostatic member 40a may be astrip of bimetal composed of laminae of AInvar, an alloy of 64 per centiron and 36 per cent nickel, and of chrome iron, an alloy of 75 per centironl 22 per cent nickel, and 3 per cent chromium, in equal thicknesses,welded together, andof `0.03 inch total thickness. Above its foot Ma, itmay be 1A@ inch wide, with a height of H inch from the top oi the i'ootto the underside of the latch portion 45a, which may project about 1A@inch at right angles to the upright portion of the member. The part 55may be of lndurated fiber about 0.1 inch thick. The` heater 43a may be ahelical coil of about 30 turns of 5.6 mil resistance wire wound 100turns per inch so as to have a final internal diameter of 0.075 inch,with end legs about 5 mm. 1ong each, giving a total wire length of 157.6mm., and a resistance of about 12.15 to 13.45 ohms when half of each' 5mm. leg is included between the effective coil connections to the partsMa and 53. (The resistance wire here referred to is an allo'y of nickel75 per cent, iron 12 per cent, chromium 11 per cent; and manganese 2 percent.) The thermal insulator may be a block formed out of a commercialsteatite ceramic mikture by extrusion, and baked. in the usual way. Itsdimensions when finished may be 3A; inch x if inch x u, inch. It isshown with one long edge rounded to a @ig inch radius and coaxial with abore of 0.08 inch diameter for the heater Ma; and with a rectangularhole parallel with said bore for the thermostat 46a, this opening being-FAM x 3,(54 inch' fore its latch end 45a is bent over as' shown in thethickness of the part 55 in Fig. 3, thus assuming the character of ablock lying directly against the base plate 20; and instead of beingmade with a bent foot 4l and securing tangs 58, 58, the thermostat 0bconsists of a mere strip whose upper end is bent at 45, while its unbentlower end is iitted and secured in a conformative hole 59h in the part55h with a.driving or force iit. While the whole-part 55h might consistof a single piece of very thick fiber, its thickness may as shown beprovided by making the spacer 5'lb of the same dimensions as the piecethat corresponds to the part 55 in Fig. 3, the two pieces being securelyclamped together and to the disc 20 by the rod 41h (which as in Fig. 3is not attached to any connector 22), and being keyed to one another bythe member 40h.

"In Figs. 2, 3, and 4, various parts and features are marked withthe/'same reference characters as the corresponding ones in earlierfigures, in order to dispense with repetitive description, a distinctiveletter being added where such distinction seems desirable.

What I claim as new and desire to secure by.

Letters Patent ofthe United States is:

l. In starting means for a discharge lamp including a starting circuitthat is interrupted to start the discharge in said lamp and alsocomprising an automatic starter to beconnected in the starting circuitfor making and breaking the same, the combination with a casing for saidstarter having an opening in its end wall, of a base member forsupporting said casing, a switch in said casing to be connected inseries with said starter for also interrupting the starting circuit,said switch comprising longitudinal support and guide means supported-by said base member independently of said automatic starter, a switchcontact and latching device slideable along said lastmentioned means, anoperator for said device movable inward and outward through the casingend wall opening, a spring for urging said device along the support andguide means toward said end wall, a thermostatic latch having switch`contact and latch means for engaging said device and retaining itagainst the action of said spring, and a heater for connection in serieswith said automatic starter for heating said thermostatic latch andreleasing it in response to heat accumulation during repeated operationof the starter.

2.l In starting means for a discharge lamp including a starting circuitthat is interrupted to start the discharge in said lamp and alsocomprising an automatic starter to be connected 1n the starting circuitfor making and breaking the same, the combination with a casing for saidstarter having an opening in its end wall, of a base member forsupporting said casing, a switch in said casing to be connected inseries with said starter for also interrupting the starting circuit,said switch comprising longitudinal support and guide means supported bysaid base member independently of said starter, a switch contact andlatching device slideable along said last-mentioned means, an operatorfor said device movable inward and outward through the casing end wallopening and having a stop shoulder for engaging said end wall to limitprotrusion of the operator through the opening, a helical compressionspring associated with the guide and supporting'means for urging saiddevice toward said end wall, a thermostat extending along adjacent saidsupport and guide means and provided with device and retaining itagainst the action of said spring, and a heater for connection in serieswith 3. In starting means for a discharge lamp including a startingcircuit that'is interrupted to start the discharge in said lamp and alsocomprising an automatic starter for making and breaking the circuit, thecombination with a .base carrying connectors to be connected to thesides of the starting Acircuit and a. casing for said starter attachedto said base and having an openl ing in its wall opposite the base, ofaswitch in said casing to be connected between said connectors throughsaid starter for also interrupting the starting circuit, said switchcomprising a longitudinal rod upstanding from said base supported bysaid base independently of s'aidv starter, a bevelled` and shoulderedlatch and 1 switch contact head slideable along said rod, an

for heating said thermostatic latch and releasing it in response to heataccumulation during repeated operation of the starter.

4. In starting means for a discharge lamp includingastarting circuitthat is interrupted 'to start the discharge in said lamp and alsocomprising an Aautomatic starter to be connected in the starting circuitfor making and breaking the same, the combination with a casing for theautomatic starter having a reentranhend wall with a substantiallycentral opening therein, of a switch in said casing to be connected inseries with said starter for also interrupting the starting circuit,said switch comprising longitudinal support and guide means supported bysaid base independently of said starter, a switch contact and latchingdevice slideable along said last-mentioned means and having a lateralportion extending substantially into line with said casing end wallopening, an operating button for said device 'movable inward and outwardthrough said opening -and having a stop shoulder for engaging said endwall to limit protrusion of vthe button through the opening, and alsohaving a portion in line with said lateral'portion of said device forengaging the same, a spring for urging said device along the support andguide means toward said end wall, a thermostatic latch having switchcontact and latch means for engaging said device and retaining itagainst the action of said spring, and a heater for connection in serieswith said automatic starter for heating said thermostatic latch andreleasing it in response to heat accumulation during repeated operationof the starter.

5. In starting means for a discharge lamp including a starting circuitthat is interrupted to start the discharge in said lamp and alsocomprising an automatic `starter for making and breaking the circuit,the combination with a base carrying connectors to be connected to thesides of the starting circuit and a casing for said starter attached tosaid base and having an opening in its wall opposite the base, of aswitch in 10 said casing to be connected between said connectorsthrough'said starter for also interrupting the starting circuit, saidswitch comprising a longitudinal rod upstanding from said base supportedby said base independently of said starter,

, a bevelled and shouldered latch and switch contact head slideablealong said rod, an operator for said head movable inward and outwardthrough said casing wall opening, a spring around said rod urging saidhead away from said base, an insulative part mounted on and projectinglaterally from said rod adjacent said base, a thermostat extendingalongside said rod anchored to said part at its end adjacent said baseand at its other end provided with switch Contact and latch means forcoacting with the head shoulder, to retain the head against the actionof said spring, and also with the head bevel, and a heater forconnection in series with said automatic starter for heatingsaid/thermostatic 'latch and releasing it in resp'on'seto heataccumulation during repeated operation of the starter.

K 6. In starting means for a discharge lamp including a starting circuitthat is interrupted to vstart the discharge in said lamp and alsocomprising an: automatic starter to be .connected in the startingcircuit for making and breaking the same, the combination with a casingfor said starter having an opening in its end Wall, of a switch in said-casing to be connected in series with said starter for alsointerrupting the startlng circuit,'said switch comprising longitudinalsupport and guide means, a switch contact and latching device slideablealong said last-mentioned means, an operator for said device movableinward and outward through the casing end wall opening, a spring forurging said device along the support and guide means toward said endwall, a thermostatic latch having switch contact and latch means forengaging said device and retaining it against the action of ,saidspring, a heater for connection in series with said automatic starterfor heating said thermostatic latch and releasing it in response to heataccumulation during repeated operation of the starter, and a commonceramic thermal insulator separately surrounding said thermostat andheater, whereby the'tripping time of the thermostatic latch is increasedrelative to the wattage taken by the heater.

'7. In starting means for a discharge lamp including a; starting circuitthat is interrupted Ato start the discharge in said lamp and alsocomprising an automatic starter to beu connected in the starting circuitfor making and breaking the same, the combination with a casing for saidstarter having an opening in its end wall, of a switch in said casing tobe connected in series with said starter for also interrupting thestarting circuit, said switch comprising longitudinal support and guidemeans, a switch contact and latching device slideable along saidlast-mentioned means, an operator for said device movable inward andoutward through the casing end wall'opening and having a stop shoulderfor engaging said end wall to limit protrusion of the operator throughthe opening, a spring for urging said device'along the support and guidemeans towardvsaid end wall, a thermostat extending along adjacent saidsupport and guide means and =provided with switch contact and latchmeans for engaging said device and retaining it against the action ofsaid spring, a resistance heater extending alongside said thermostat forconnection in series with said automatic starter for heating 11 saidthermostatic latch and releasing it in response to heat; accumulationduring repeated operations of the starter, and a common thermalinsulator enclosing said thermostat and heater in thermo-insulativelyseparated parallel recesses,

whereby the tripping time of the thermostatic latch is increasedrelative to the wattage takeny by the heater.

MARK A. TOW'NSEND.

REFERENCES CITED The following references are of record in the file ofthis patent:

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